Plants less than 2.5 mm. Leaves broadly linear to lanceolate, (0.8--)1.4(--2.5) ´ (0.15--)0.2(--0.4) mm; margins serrulate to strongly serrate distal to the proximal third; apex slenderly acuminate, papillose; costa not always apparent at the base, often filling the acumen, percurrent; areolation compact distally; median laminal cells smooth or slightly papillose; distal laminal cells somewhat papillose. Capsule with very few stomates, mostly near the base. Spores spherical or reniform, (43--)75(--107) ´ (35--)50(--75) µm, orange-brown.

Capsules mature year around. Moist or drying disturbed soil; 7--1000 m; Ont., Que., Sask.; Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kan., Ky., La., Md., Mich., Minn., Miss., Mo., Neb., N.H., N.J., N.Y., N.C., Ohio, Oreg., Pa., Tenn., Tex., Va., W.Va., Wis.; Europe (Germany); Asia (Japan); Pacific Islands (New Zealand).

Ephemerum crassinervium var. papillosum (Austin) Renauld & Cardot has not been considered worthy of recognition as a variety of E. crassinervium (A. J. Grout 1935; V. S. Bryan and L. E. Anderson 1957), a conclusion sustained by the present studies. It was described originally because of its narrower leaves and a strongly papillose calyptra, but both characters have been found to vary independently. Ephemerum crassinervium var. crassinervium shares several characters with Ephemerum sessile (Bruch & Schimper) K. J. A. Müller, a relatively common species in Europe, the Mediterranean islands, and north Africa. It was reported as occurring in North America (W. S. Sullivant 1856), and plants labeled Ephemerum sessile from “central Ohio” were distributed by Sullivant and Lesquereux in 1856 as number 21 of Musci Boreali-Americani. In 1933 A. J. Grout in Mosses of North America commented that number 21 of Musci Boreali-Americani is not E. sessile, and I have likewise found no North American plants that are convincingly E. sessile, as distinct from E. crassinervium.